1. Field of the Invention
The present invention relates to percussion instruments of the type suitable for use in dental practice for determining the degree of looseness of a tooth, and in particular to such a percussion instrument having a ram displacably seated in a handpiece and a means for accelerating the ram to a specific velocity from an initial position, the ram subsequently moving toward a test object in free flight decoupled from its drive means at a constant velocity, and a magnetic means for returning the ram to its initial position.
2. Description of the Prior Art
A percussion instrument having a displacably seated ram and a means for accelerating the ram to a specific velocity is disclosed in German OS No. 2,617,779. In this known instrument, the ram is mounted in a housing so as to move longitudinally back and forth in the axial direction of the instrument. The ram is accelerated to a specific velocity by means of a helical spring disposed in a front portion of the instrument, near the free end of the ram carrying a test head. The ram is maintained in its initial position by means of a magnetic coil. After complete relaxation of the spring, the ram separates from the spring and, guided by bearings, approaches the object to be tested with a theoretically constant velocity in free flight. After impact against the object to be tested, the ram is repelled in a direction toward its initial position by the rebound or reactive force arising upon impact. The coil is supplied with a current pulse toward the end of the return motion of the ram such that the ram is returned to its initial position by the magnetic field generated thereby, and the spring is again tensed. An acceleration pickup is connected to the ram by means of a flexturally slack cable for detecting a change in velocity of the ram upon impact of the ram against the object to be tested. This change in velocity is evaluated during the excursion and return paths of the ram for specific test and diagnostic purposes. The velocity change is evaluated in an electronic evaluation unit connected to the acceleration pickup. In dental practice, for example, the velocity of the ram is a direct indicator of the degree of tooth mobility, that is, the degree to which a tooth is loosened in the gum tissue. The time within which the tooth to be tested returns to its initial position after the impact pulse provides useful information regarding the condition of the tooth retaining means.
Conventional percussion instruments of the type described in German OS No. 2,617,779 have several disadvantages. Because the ram executes a substantially straight line flight motion proceeding in the axial direction of the instrument, the velocity of the ram will assume a theoretically constant value only when the instrument is held substantially horizontal so that the ram velocity is not influenced by gravitational force. When the attitude of the instrument departs from the horizontal, the flight velocity of the ram is altered and the measuring results are correspondingly falsified. The only safeguard available to avoid such data falsification in the use of conventional percussion instruments is to hold the instrument absolutely horizontally, requiring a high degree of concentration from the user.
A further disadvantage of such conventional percussion instruments is that the test objects such as, for example, a patient's teeth, can only be subjected to impact from that side of the patient's mouth from which the instrument can be introduced at a horizontal attitude. Test subjects which are not accessible in the direction in which the instrument is introduced to the object to be tested horizontally can therefore not be measured, or at least cannot be reliably measured. In the use of the instrument for examining tooth mobility, teeth in the molar region cannot be reached with a conventional percussion instrument or, if accessible, cannot be accurately tested.
Another disadvantage in conventional percussion instruments of the type described above is the arrangement of the acceleration pickup which generates acceleration signals for determining the ram velocity. The acceleration pickup utilized in conventional percussion instruments is disposed at that end of the ram facing away from the test head and is connected to evaluation electronics by means of a highly flexible lead. Because the lead must participate in the ram movement, that is, be comovable therewith, tearing of the line can occur. Moreover, the acceleration signal obtained with conventional acceleration pickups of this type is relatively small and must be amplified by means of relatively complex circuitry. A further disadvantage is that the ram in conventional devices is seated in common bearings and is decelerated by the frictional forces of the bearings. These frictional forces may vary over time by the action of external influences such as humidity, dust or the like. Thus, the ram velocity is subject to corresponding varying alterations which falsify the test results.